1. Field of the Invention
The present invention relates to a developing apparatus used with an image forming apparatus of electrophotographic or electrostatic type such as a copying machine, a printer and the like and adapted to develop a latent image an an image bearing member.
2. Related Background Art
In conventional image forming apparatuses of electrophotographic type, a latent image is formed on a photosensitive drum as an image bearing member by selectively exposing a surface of the photosensitive drum uniformly charged by a charging apparatus by means of an exposing apparatus, and the latent image is visualized with toner as a developer by a developing apparatus to form a toner image, and the toner image is transferred onto a recording medium such as a paper or an OHP sheet, and the toner image is fixed to the recording material with heat and pressure, thereby obtaining an image.
On the other hand, after the toner image is transferred, residual toner remaining on the photosensitive drum is removed by a cleaning apparatus, for preparing for next image formation starting from charging.
Incidentally, in such image forming apparatuses, when the apparatus is used for a long term, replenishing of new toner to the developing apparatus and adjustment, cleaning and exchanging of various process parts including the photosensitive drum are required. It was practically difficult for the users other than experts to perform such maintenance. Thus, in order to solve this problem, there has been proposed a process cartridge in which a toner and process means such as a photosensitive drum, a developing apparatus, charging apparatus and cleaning apparatus are integrally incorporated as a cartridge unit which can detachably be mounted to an image forming apparatus, thereby enhancing maintenance ability, and such a process cartridge is put to practical use.
The developing apparatus included in such a process cartridge comprises a developing apparatus frame for containing toner as developer, a developer carrying member, and a developing blade as a developer regulating member.
The developer carrying member comprises a developing roller as a hollow cylindrical member in which flanges as shaft members are attached to both ends of a hollow cylinder made of conductive and nonmagnetic material such as aluminum, and a magnet roller held within the developing roller. A gear for transmitting a rotational driving force to the developing roller is secured to one of the flanges of the developing roller. The developer carrying member is arranged so that an outer peripheral surface of the developer roller cylinder is opposed to an outer peripheral surface of the photosensitive drum with a predetermined gap (preferably, 0.2 to 0.5 mm) therebetween.
The developing blade has one end secured to the developing apparatus frame and the other or free end urged against the outer peripheral surface of the developing roller cylinder.
The interior of the developing apparatus frame is divided into a toner chamber and a developer supplying chamber, and the toner as developer is contained in the toner chamber, and the developer carrying member and the developing blade are contained in the developer supplying chamber.
The toner in the toner chamber is carried from the toner chamber to the developer supplying chamber by its own weight or by conveying means and is attracted to the vicinity of the developer carrying member by a magnetic force of the magnet roller. During image formation, the developing roller is rotated by the driving force transmitted to the flange gear at its one end. The toner attracted around the outer peripheral surface of the developing roller is carried toward the developing blade as the developing roller is rotated, and regulation of a developer layer thickness and application of tribo-electricity are effected by the developing blade while the toner is being carried toward the photosensitive drum.
As the developer carrying members, there have been proposed a developer carrying member in which, as shown in FIG. 7, magnet shafts (first magnet shaft 272 and a second magnet shaft 273) as shaft portions provided on both ends of a magnetizing portion 271 as a main part of a magnet roller 270 as a magnetic field generating member extend through holes formed in flanges 262, 263 as shaft members of a developing roller 260 as a hollow cylindrical member and are supported by a developing apparatus frame 246, and a developer carrying member in which, as shown in FIGS. 8 and 9, one (first magnet shaft 172) of magnet shafts as shaft portions is held within an inner diameter portion of one (first flange 162) of flanges as shaft members and only the other magnet shaft (second magnet shaft 173) extends through a through hole formed in the other flange (second flange 163) and is held by a developing apparatus frame 146,and these developer carrying members are put to practical use.
Next, the developer carrying members will be fully described.
First of all, the cylinder-shaped developer carrying member 261 in which the shafts at both ends of the magnet roller are held by the developing apparatus frame 246 will be explained with reference to FIG. 7.
As mentioned above, the developing roller 260 as the hollow cylindrical member which forms a main part of the developer carrying member 261 includes a cylinder portion 261 formed from aluminum, and the first and second flanges 262, 263 as the shaft members attached to both ends of the cylinder portion 261 and is rotatably held by the first and second flanges 262, 263.
A gear 264 as drive transmitting means is secured to the first flange 262.
The magnet roller 270 as the magnetic field generating means disposed within the developing roller 260 has the magnetizing portion 271 having a plurality of magnetic poles, and the first and second magnet shafts 272, 273 as the shaft portions provided on both ends of the magnetizing portion.
The first and second magnet shafts 272, 273 extend through the first and second flanges 262, 263 of the developing roller 260, and at least one of the magnet shafts is provided at its tip end area with D-cut shaped rotation preventing means so that the magnet roller 270 is always held at a predetermined angle in a circumferential direction.
If the first and second flanges 262, 263 of the developing roller 260 are misaligned with the cylinder portion 261 in an axial direction, since the developing roller 260 is shifted by an amount corresponding to the misalignment amount during one revolution thereof, an abutting condition between the developing roller and the developing blade 242 will be changed, with the result that uniformity of the image cannot be maintained. On the other hand, since a temperature within the main body of the apparatus is increased as the apparatus is operated, operating environmental humidity of the developing apparatus is not constant. Thus, in order to maintain the concentricity between the first and second flanges 262, 263 and the cylinder portion 261 with high accuracy regardless of the environmental temperature, a technique in which at least one of the first and second flanges 262, 263 is formed from aluminum similar to the cylinder portion 261 and the aluminum flange is press-fitted in the cylinder portion 261 and the first and second flanges 262, 263 due to difference in thermal expansion caused by the temperature increase is prevented. Incidentally, in this case, the gear 264 is generally secured to the aluminum flange.
Next, the cylinder-shaped developer carrying member 161 in which one (first magnet shaft 172) of the magnet roller 170 is held within the inner diameter portion of one (first flange 162) of flanges and the second magnet shaft 173 is held by the developing apparatus frame 146 will be explained with reference to FIG. 8.
Also in the developing roller 160 shown in FIGS. 8 and 9, similar to the developing roller 260 shown in FIG. 8, it is preferable that one (first flange 162 in the illustrated example) of the flanges is formed from aluminium. On the other hand, although material obtained by adding ferrite to nylon has widely been used for making the magnet roller 170, since a sliding property of such material with respect to metal is not good, a magnet supporting member 180 as a bearing member formed from material (for example, resin such as polyacetal) which has a good sliding property is used between the magnet shaft (first magnet shaft 172 in the illustrated example) and the first flange 162.
Incidentally, since there is difference in coefficient of linear expansion between the aluminum forming the first flange 162 and the resin forming the magnet supporting member 180 and the resin (nylon) forming the magnet roller 170, a gap is previously set between the first flange 162 and the magnet supporting member 180 or between the magnet supporting member 180 and the magnet shaft 172 in consideration of such coefficient of linear expansion.
By the way, as image forming apparatuses have recently been progressed increasingly, the entire image forming apparatus (main body of the apparatus) and the process cartridge are requested to be made more compact. In order to realize this, the diameter of the developing roller must be reduced. Further, in order to make the process cartridge compact, the arrangement, as shown in FIGS. 8 and 9, in which one of the magnet shafts of the magnet roller is held by the flange portion (first flange) of the developing roller is inevitable.
However, in the developer carrying member 161, if the magnet roller 170 is vibrated during the image formation, since the magnetic force (magnetic flux density) of the magnet roller 170 acting on a developing area where the developing roller 160 is opposed to the photosensitive drum is changed, the magnetic force for holding the toner on the surface of the developing roller 160 in the developing area is also changed, with the result that the image density is changed. For example, it was experimentally found that in case of a magnet roller 170 having four magnetic poles, a relationship between a distance d from the surface of the magnet roller 170 to the surface of the developing roller 160 and a magnetic force f on the surface of the developing roller 160 is as shown in FIG. 10 and a relationship between a magnetic force f acting on the surface of the developing roller 160 and image density z at predetermined print ratio (half toner) is as shown in FIG. 11.
Thus, the following factors relating to the positional accuracy of the magnet roller 170 affect an influence upon the uniformity of the image density:
(1) the deviation of the concentricity between the inner diameter of the magnet supporting member and the outer diameter of the magnet supporting member; and
(2) the deviation of the concentricity between the outer diameter of the developing roller and the inner diameter of the flange portion.
An object of the present invention is to provide a developing apparatus in which an axial direction of a developer carrying member is reduced.
Another object of the present invention is to provide a developing apparatus in which positional accuracy of a magnet roller is high.
A further object of the present invention is to provide a developing apparatus comprising a developer carrying member opposed to an image bearing member for bearing a latent image and adapted to carry magnetic developer, a magnet provided within the developer carrying member and having one end secured to a frame of the apparatus, and a bearing fitted on other end of the magnet and adapted to rotatably support the developer carrying member, wherein a fitted portion between the magnet and the bearing has a noncircular shape.
The other objects and features of the present invention will be apparent from the following detailed explanation of the invention referring to the accompanying drawings.